refactor: rewrite message creation (#2774)

* Rework createMessage
- MessageAttachment is now structurally similar to FileOptions
- No longer mutates the object passed as options
- Supports more permutations of arguments

* Ignore complexity warning

* Refactor name finding

* Fix typo

* Update typings

* Default name to null for MessageAttachment

* Make Message#reply use transformOptions

* Move transformOptions

* Fix Message#reply

* Fix mutation

* Update tests

* Fix options passing

* Refactor into APIMessage

* Fix webhook send

* Expose APIMessage

* Add documentation

* Add types

* Fix type doc

* Fix another type doc

* Fix another another type doc (is this one even right!?)

* Remove trailing comma

* Properly clone split options

* Add support for sending file as stream

* Missed a doc

* Resolve files only once when splitting messages

* This looks nicer

* Assign directly

* Don't cache data and files

* Missing return type

* Use object spread instead Object.assign

* Document constructors

* Crawl is a little dot

* comp pls

* tests: sanitize local file path, disable no-await-in-loop

src/structures/MessageAttachment.js

@@ -2,77 +2,41 @@ const Util = require('../util/Util');
 
 /**
  * Represents an attachment in a message.
- * @param {BufferResolvable|Stream} file The file
- * @param {string} [name] The name of the file, if any
  */
 class MessageAttachment {
-  constructor(file, name, data) {
-    this.file = null;
+  /**
+   * @param {BufferResolvable|Stream} attachment The file
+   * @param {string} [name=null] The name of the file, if any
+   * @param {Object} [data] Extra data
+   */
+  constructor(attachment, name = null, data) {
+    this.attachment = attachment;
+    this.name = name;
     if (data) this._patch(data);
-    if (name) this.setAttachment(file, name);
-    else this._attach(file);
   }
 
   /**
-    * The name of the file
-    * @type {?string}
-    * @readonly
-    */
-  get name() {
-    return this.file.name;
-  }
-
-  /**
-    * The file
-    * @type {?BufferResolvable|Stream}
-    * @readonly
-    */
-  get attachment() {
-    return this.file.attachment;
-  }
-
-  /**
-    * Sets the file of this attachment.
-    * @param {BufferResolvable|Stream} file The file
-    * @param {string} name The name of the file
-    * @returns {MessageAttachment} This attachment
-    */
-  setAttachment(file, name) {
-    this.file = { attachment: file, name };
+   * Sets the file of this attachment.
+   * @param {BufferResolvable|Stream} attachment The file
+   * @param {string} [name=null] The name of the file, if any
+   * @returns {MessageAttachment} This attachment
+   */
+  setFile(attachment, name = null) {
+    this.attachment = attachment;
+    this.name = name;
     return this;
   }
 
   /**
-    * Sets the file of this attachment.
-    * @param {BufferResolvable|Stream} attachment The file
-    * @returns {MessageAttachment} This attachment
-    */
-  setFile(attachment) {
-    this.file = { attachment };
-    return this;
-  }
-
-  /**
-    * Sets the name of this attachment.
-    * @param {string} name The name of the image
-    * @returns {MessageAttachment} This attachment
-    */
+   * Sets the name of this attachment.
+   * @param {string} name The name of the file
+   * @returns {MessageAttachment} This attachment
+   */
   setName(name) {
-    this.file.name = name;
+    this.name = name;
     return this;
   }
 
-  /**
-    * Sets the file of this attachment.
-    * @param {BufferResolvable|Stream} file The file
-    * @param {string} name The name of the file
-    * @private
-    */
-  _attach(file, name) {
-    if (typeof file === 'string') this.file = file;
-    else this.setAttachment(file, name);
-  }
-
   _patch(data) {
     /**
      * The ID of this attachment